This invention relates to motor starting circuits and more specifically relates to a control circuit for controlling the energization of the starting capacitor and starting winding of a single phase motor.
Single phase AC induction motors have a subclass that utilize a starting winding, and sometimes, a starting capacitor in series with the winding to generate a rotating magnetic field. In most such motors the starting capacitor and the starting winding circuit is cut off from the supply after the motor reaches about 75 to 80% of its rated speed, when the motor main winding alone provides sufficient operating torque. If the supply voltage remains connected to the start winding and the capacitor, the capacitor and the winding need to be significantly larger and more robust, and thus more expensive. Therefore, most such motors have a switch that turns off the supply voltage to this branch after starting.
This added switch is expensive, slow and bulky. It would be desirable to provide a high speed, inexpensive and reliable control arrangement for disconnecting the starting winding and starting capacitor from the power line when the main motor winding reaches a predetermined energization from the power line.
In accordance with the invention, a current sensor is coupled to the main motor winding and is arranged to put out a signal which is indicative of the energization state of the main winding, for example, a measure of the current in the main winding. A controllably conductive solid state switching device such as a triac, a bipolar transistor or pairs of antiparallel connected SCRs, power MOSFETs or IGBTs are then connected in series with the starting winding and are operated into and out of conduction by the current sensor in such a way that the device is switched on during motor starting, but is switched off when the main motor winding is energized sufficiently as to provide its own operating torque.
Thus, the sensor output must be high at low motor current to turn on the controllably conductive device, but its output must decrease as the motor current increases to its self-sustaining value, at which point the controllably conductive device turns off to open the starting winding circuit. In this way, the starting winding and starting capacitor can be reduced to a small fraction of their required size if they were permanently left in the circuit.
In accordance with the invention, the current sensor may be made of a negative temperature coefficient (NTC) resistor or a saturable current transformer in series with the main winding; or a resistor network which carries the locked rotor current of the main winding. Other similar devices which will produce a desired preprogramed output can also be used.
In each case, the novel circuits require low volume and require little power and are highly reliable and inexpensive.